This invention relates generally to the field of testing systems and methods, and more particularly to systems and methods for transport or permeation testing. In one particular aspect, the invention provides systems and methods for culturing cells onto a membrane and then using the membrane to simulate an epithelial cell layer, such as the cells which form the inner lining of a human intestine, the blood-brain barrier or blood vessels. In this way, transport or permeation tests may be performed using the membrane.
In humans, ingested food passes from the stomach to the small intestine where proteins, fats, carbohydrates and other nutrients are absorbed and distributed into circulation for use in various organs and cells throughout the body. The small intestine is about five to six meters in length and has an extremely large surface area for absorbing nutrients and other materials. The interior of the small intestine includes the mucosal epithelium which comprises small fingerlike projections called villi which protrude into the intestinal lumen and provide the nutrient absorption surface.
For a variety of reasons, it is desirable to study and evaluate how various drugs and other chemicals which are orally ingested by a human will be absorbed into the blood stream through the intestinal wall. Such evaluation can be useful in, for example, drug testing to determine how various drugs would permeate through the intestinal wall and be absorbed into the blood stream after being orally ingested. Determining transport of various substances through other types of epithelial cells can also be useful in therapeutically treating patients.
In order to evaluate how certain chemicals or other substances will permeate epithelial cells, some have proposed growing mammalian-based cells on a membrane which in turn is used to mimic a cell layer within the body. Some previously proposed testing systems comprise a cup having a membrane at its bottom end. After the cells have grown onto the membrane, the cup is inserted into a larger cup or well and various chemicals are placed into the upper cup to evaluate how the chemicals will permeate the cells on the membrane and enter into fluid in the bottom well.
Such testing systems suffer from a variety of drawbacks, including the significant amount of time required to separately seed the cells into each of the upper cups and to add and replace cell culture nutrients in each cup at regular intervals. A further drawback to such systems is their limited use in accommodating smaller sized membranes. For example, many multi-well plates are being provided with increased numbers of wells whose dimensions are significantly smaller to create larger densities of wells within the plates. Accordingly, each upper cup and its membrane needs to be made smaller in order to fit within the smaller wells. However, when reducing the size of the membranes with the testing systems described above, the membrane's surface area may be too small to provide an adequate transport interface. In turn, this can lower concentrations or transported amounts to levels which restrict analytical methodologies presently available to quantify results. Further, the activity provided by a cell layer on such small membrane sizes may not be representative of the activity provided by a cell layer on a larger membrane.
Hence, for these and other reasons, it would be desirable to provide systems and methods which will allow cells to be seeded and cultured in an efficient manner. Further, it would be desirable to provide a design for a testing system where membrane densities are greatly increased while still being sufficiently sized to effectively accommodate cell growth and to provide an adequate transport interface.